It’s possible, but if you had a full world’s worth of assets loaded and running, the game would either run at an unplayable frame rate or would be significantly reduced in visual fidelity. This kind of thing actually did happen way back in the day - we used to load the entire game into memory in the arcade coin-operated machines and cartridge days. Nowadays we can do a lot with streaming data from disk into memory, most of which comes from being clever in optimizing what gets loaded when. As long as we can keep the smoke and mirrors going, the player never sees the railroad tracks they’re following.
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